Photovoltaic devices and method of making

ABSTRACT

A photovoltaic device is presented. The photovoltaic device includes a layer stack; and an absorber layer is disposed on the layer stack. The absorber layer comprises selenium, wherein an atomic concentration of selenium varies across a thickness of the absorber layer. The photovoltaic device is substantially free of a cadmium sulfide layer.

BACKGROUND

The invention generally relates to photovoltaic devices. Moreparticularly, the invention relates to photovoltaic devices includingselenium, and methods of making the photovoltaic devices.

Thin film solar cells or photovoltaic (PV) devices typically include aplurality of semiconductor layers disposed on a transparent substrate,wherein one layer serves as a window layer and a second layer serves asan absorber layer. The window layer allows the penetration of solarradiation to the absorber layer, where the optical energy is convertedto usable electrical energy. The window layer further functions to forma heterojunction (p-n junction) in combination with an absorber layer.Cadmium telluride/cadmium sulfide (CdTe/CdS) heterojunction-basedphotovoltaic cells are one such example of thin film solar cells, whereCdS functions as the window layer.

However, thin film solar cells may have low conversion efficiencies.Thus, one of the main focuses in the field of photovoltaic devices isthe improvement of conversion efficiency. Absorption of light by thewindow layer may be one of the phenomena limiting the conversionefficiency of a PV device. Further, a lattice mismatch between thewindow layer and absorber layer (e.g., CdS/CdTe) layer may lead to highdefect density at the interface, which may further lead to shorterinterface carrier lifetime. Thus, it is desirable to keep the windowlayer as thin as possible to help reduce optical losses by absorption.However, for most of the thin-film PV devices, if the window layer istoo thin, a loss in performance can be observed due to low open circuitvoltage (V_(OC)) and fill factor (FF).

Thus, there is a need for improved thin film photovoltaic devicesconfigurations, and methods of manufacturing these.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments of the present invention are included to meet these andother needs. One embodiment is a photovoltaic device. The photovoltaicdevice includes a layer stack; and an absorber layer is disposed on thelayer stack. The absorber layer comprises selenium, wherein an atomicconcentration of selenium varies across a thickness of the absorberlayer. The photovoltaic device is substantially free of a cadmiumsulfide layer.

One embodiment is a photovoltaic device. The photovoltaic deviceincludes a layer stack and an absorber layer disposed on the layerstack. The layer stack includes a transparent conductive oxide layerdisposed on a support and a buffer layer disposed on the transparentconductive oxide layer. Alternatively, the layer stack includes atransparent conductive oxide layer disposed on a support, a buffer layerdisposed on the transparent conductive oxide layer, and an interlayerdisposed on the buffer layer. The absorber layer is disposed directly incontact with the layer stack, wherein the absorber layer comprisesselenium, and wherein an atomic concentration of selenium varies acrossa thickness of the absorber layer.

One embodiment is a method of making a photovoltaic device. The methodincludes providing an absorber layer on a layer stack, wherein theabsorber layer comprises selenium, and wherein an atomic concentrationof selenium varies across a thickness of the absorber layer. Thephotovoltaic device is substantially free of a cadmium sulfide layer.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings,wherein:

FIG. 1 is a schematic of a photovoltaic device, according to someembodiments of the invention.

FIG. 2 is a schematic of a photovoltaic device, according to someembodiments of the invention.

FIG. 3 is a schematic of a photovoltaic device, according to someembodiments of the invention.

FIG. 4 is a schematic of a photovoltaic device, according to someembodiments of the invention.

FIG. 5 is a schematic of a photovoltaic device, according to someembodiments of the invention.

FIG. 6 is a schematic of a photovoltaic device, according to someembodiments of the invention.

FIG. 7 is a schematic of a method of making a photovoltaic device,according to some embodiments of the invention.

FIG. 8 shows the performance parameters for photovoltaic devices,according to some embodiments of the invention.

FIG. 9 shows the time-resolved photoluminescence (TRPL) lifetime curvesfor photovoltaic devices, according to some embodiments of theinvention.

FIG. 10 shows the secondary ion mass spectrometry (SIMS) profile forphotovoltaic devices, according to some embodiments of the invention.

FIG. 11 shows the selenium to telenium ratio for photovoltaic devices,according to some embodiments of the invention.

FIG. 12 shows the external measured external quantum efficiency (EQE)for photovoltaic devices, according to some embodiments of theinvention.

FIG. 13 shows the PL spectral comparison for photovoltaic devices,according to some embodiments of the invention.

DETAILED DESCRIPTION

As discussed in detail below, some of the embodiments of the inventioninclude photovoltaic devices including selenium.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about”, and “substantially” is not to be limited tothe precise value specified. In some instances, the approximatinglanguage may correspond to the precision of an instrument for measuringthe value. Here and throughout the specification and claims, rangelimitations may be combined and/or interchanged, such ranges areidentified and include all the sub-ranges contained therein unlesscontext or language indicates otherwise.

In the following specification and the claims, the singular forms “a”,“an” and “the” include plural referents unless the context clearlydictates otherwise. As used herein, the term “or” is not meant to beexclusive and refers to at least one of the referenced components (forexample, a layer) being present and includes instances in which acombination of the referenced components may be present, unless thecontext clearly dictates otherwise.

The terms “transparent region” and “transparent layer” as used herein,refer to a region or a layer that allows an average transmission of atleast 70% of incident electromagnetic radiation having a wavelength in arange from about 350 nm to about 1000 nm.

As used herein, the term “layer” refers to a material disposed on atleast a portion of an underlying surface in a continuous ordiscontinuous manner. Further, the term “layer” does not necessarilymean a uniform thickness of the disposed material, and the disposedmaterial may have a uniform or a variable thickness. Furthermore, theterm “a layer” as used herein refers to a single layer or a plurality oflayers, unless the context clearly dictates otherwise.

As used herein, the term “disposed on” refers to layers disposeddirectly in contact with each other or indirectly by having interveninglayers therebetween, unless otherwise specifically indicated. The term“adjacent” as used herein means that the two layers are disposedcontiguously and are in direct contact with each other.

In the present disclosure, when a layer is being described as “on”another layer or substrate, it is to be understood that the layers caneither be directly contacting each other or have one (or more) layer orfeature between the layers. Further, the term “on” describes therelative position of the layers to each other and does not necessarilymean “on top of” since the relative position above or below depends uponthe orientation of the device to the viewer. Moreover, the use of “top,”“bottom,” “above,” “below,” and variations of these terms is made forconvenience, and does not require any particular orientation of thecomponents unless otherwise stated.

As discussed in detail below, some embodiments of the invention aredirected to a photovoltaic device including selenium. A photovoltaicdevice 100, according to some embodiments of the invention, isillustrated in FIGS. 1-5 . As shown in FIGS. 1-5 , the photovoltaicdevice 100 includes a layer stack 110 and an absorber layer 120 disposedon the layer stack 110. The absorber layer 120 includes selenium, and anatomic concentration of selenium varies across a thickness of theabsorber layer 120. The photovoltaic device is substantially free of acadmium sulfide layer.

The term “substantially free of a cadmium sulfide layer” as used hereinmeans that a percentage coverage of the cadmium sulfide layer (ifpresent) on the underlying layer (for example, the interlayer or thebuffer layer) is less than 20 percent. In some embodiments, thepercentage coverage is in a range from about 0 percent to about 10percent. In some embodiments, the percentage coverage is in a range fromabout 0 percent to about 5 percent. In certain embodiments, thephotovoltaic device is completely free of the cadmium sulfide layer.

The term “atomic concentration” as used in this context herein refers tothe average number of selenium atoms per unit volume of the absorberlayer. The terms “atomic concentration” and “concentration” are usedherein interchangeably throughout the text. The term “varies across thethickness” as used herein means that the concentration of seleniumchanges across the thickness of the absorber layer in a continuous or anon-continuous manner.

In some embodiments, there is a step-change in the concentration ofselenium across the thickness of the absorber layer 120. In someembodiments, the concentration of selenium varies continuously acrossthe thickness of the absorber layer 120. Further, in such instances, thevariation in the selenium concentration may be monotonic ornon-monotonic. In some instances, the rate-of-change in concentrationmay itself vary through the thickness, for example, increasing in someregions of the thickness, and decreasing in other regions of thethickness. Moreover, in some instances, the selenium concentration mayremain substantially constant for some portion of the thickness. Theterm “substantially constant” as used in this context means that thechange in concentration is less than 5 percent across that portion ofthe thickness.

In some embodiments, the selenium concentration decreases across thethickness of the absorber layer 120, in a direction away from the layerstack 110. In some embodiments, the selenium concentration monotonicallydecreases across the thickness of the absorber layer 120, in a directionaway from the layer stack 110. In some embodiments, the seleniumconcentration continuously decreases across a certain portion of theabsorber layer 120 thickness, and is further substantially constant insome other portion of the absorber layer 120 thickness.

In certain embodiments, the absorber layer 120 includes a varyingconcentration of selenium such that there is higher concentration ofselenium near the front interface (interface closer to the frontcontact) relative to the back interface (interface closer to the backcontact).

In certain embodiments, the band gap in the absorber layer 120 may varyacross a thickness of the absorber layer 120. In some embodiments, theconcentration of selenium may vary across the thickness of the absorberlayer 120 such that the band gap near the front interface is lower thanthe band gap near the back interface.

In certain embodiments, the absorber layer 120 may include aheterojunction. As used herein, a heterojunction is a semiconductorjunction that is composed of layers/regions of dissimilar semiconductormaterial. These materials usually have non-equal band gaps. As anexample, a heterojunction can be formed by contact between a layer orregion of one conductivity type with a layer or region of oppositeconductivity, e.g., a “p-n” junction.

As will be appreciated by one of ordinary skill in the art, by varyingthe concentration of selenium in the absorber layer 120, a particularregion of the absorber layer 120 may be rendered n-type and anotherregion of the absorber layer 120 may be rendered p-type. In certainembodiments, the absorber layer 120 includes a “p-n” junction. Withoutbeing bound by any theory, it is believed that the “p-n” junction may beformed between a plurality of regions of the absorber layer 120 havingdifferent band gaps. Further, the lower band gap material may enhanceefficiency through photon confinement. In some embodiments, the absorberlayer 120 may for form a p-n junction with the underlying buffer layer113.

Without being bound by any theory, it is believed that the variation inselenium concentration may allow for a p-n junction within the absorberlayer, thus precluding the use of a separate junction forming windowlayer, such as, a CdS layer. As described earlier, the thickness of thewindow layer is typically desired to be minimized in a photovoltaicdevice to achieve high efficiency. With the presence of the varyingconcentration of selenium in the absorber layer, the thickness of thewindow layer (e.g., CdS layer) may be reduced or the window layer may beeliminated, to improve the performance of the present device. Moreover,the present device may achieve a reduction in cost of production becauseof the use of lower amounts of CdS or elimination of CdS.

Further, higher concentration of selenium near the front interfacerelative to the back interface may allow for a higher fraction ofincident radiation to be absorbed in the absorber layer 120. Moreover,Se may improve the passivation of grain boundaries and interfaces, whichcan be seen through higher bulk lifetime & reduced surfacerecombination.

The absorber layer 120 further includes a plurality of grains separatedby grain boundaries. In some embodiments, an atomic concentration ofselenium in the grain boundaries is higher than the atomic concentrationof selenium in the grains. In some embodiments, a ratio of the averageatomic concentration of selenium in the grain boundaries to the averageatomic concentration of selenium in the grains is greater than about 2.In some embodiments, a ratio of the average atomic concentration ofselenium in the grain boundaries to the average atomic concentration ofselenium in the grains is greater than about 5. In some embodiments, aratio of the average atomic concentration of selenium in the grainboundaries to the average atomic concentration of selenium in the grainsis greater than about 10.

In some embodiments, as indicated in FIG. 2 , the absorber layer 120includes a first region 122 and a second region 124. As illustrated inFIG. 2 , the first region 122 is disposed proximate to the layer stack110 relative to the second region 124. In some embodiments, an averageatomic concentration of selenium in the first region 122 is greater thanan average atomic concentration of selenium in the second region 124.

In some embodiments, the selenium concentration in the first region 122,the second region 124, or both the regions may further vary across thethickness of the respective regions. In some embodiments, the seleniumconcentration in the first region 122, the second region 124, or boththe regions may continuously change across the thickness of therespective regions. As noted earlier, in some instances, therate-of-rate-of-change in concentration may itself vary through thefirst region 122, the second region 124, or both the regions, forexample, increasing in some portions, and decreasing in other portions.

In some embodiments, the selenium concentration in the first region 122,the second region 124, or both the regions may be substantially constantacross the thickness of the respective regions. In some otherembodiments, the selenium concentration may be substantially constant inat least a portion of the first region 122, the second region 124, orboth the regions. The term “substantially constant” as used in thiscontext means that the change in concentration is less than 5 percentacross that portion or region.

The absorber layer 120 may be further characterized by the concentrationof selenium present in the first region 122 relative to the secondregion 124. In some embodiments, a ratio of the average atomicconcentration of selenium in the first region 122 to the average atomicconcentration of selenium in the second region 124 is greater than about2. In some embodiments, a ratio of the average atomic concentration ofselenium in the first region 122 to the average atomic concentration ofselenium in the second region 124 is greater than about 5. In someembodiments, a ratio of the average atomic concentration of selenium inthe first region 122 to the average atomic concentration of selenium inthe second region 124 is greater than about 10.

The first region 122 and the second region 124 may be furthercharacterized by their thickness. In some embodiments, the first region122 has a thickness in a range from about 1 nanometer to about 5000nanometers. In some embodiments, the first region 122 has a thickness ina range from about 100 nanometers to about 3000 nanometers. In someembodiments, the first region 122 has a thickness in a range from about200 nanometers to about 1500 nanometers. In some embodiments, the secondregion 124 has a thickness in a range from about 1 nanometer to about5000 nanometers. In some embodiments, the second region 124 has athickness in a range from about 100 nanometers to about 3000 nanometers.In some embodiments, the second region 124 has a thickness in a rangefrom about 200 nanometers to about 1500 nanometers.

Referring again to FIG. 2 , in some embodiments, the first region 122has a band gap that is lower than a band gap of the second region 124.In such instances, the concentration of selenium in the first region 122relative to the second region 124 may be in a range such that the bandgap of the first region 122 is lower than the band gap of the secondregion 124.

Selenium may be present in the absorber layer 120, in its elementalform, as a dopant, as a compound, or combinations thereof. In certainembodiments, at least a portion of selenium is present in the absorberlayer in the form of a compound. The term “compound”, as used herein,refers to a macroscopically homogeneous material (substance) consistingof atoms or ions of two or more different elements in definiteproportions, and at definite lattice positions. For example, cadmium,tellurium, and selenium have defined lattice positions in the crystalstructure of a cadmium selenide telluride compound, in contrast, forexample, to selenium-doped cadmium telluride, where selenium may be adopant that is substitutionally inserted on cadmium sites, and not apart of the compound lattice

In some embodiments, at least a portion of selenium is present in theabsorber layer 120 in the form of a ternary compound, a quaternarycompound, or combinations thereof. In some embodiments, the absorberlayer 120 may further include cadmium and tellurium. In certainembodiments, at least a portion of selenium is present in the absorberlayer in the form of a compound having a formula CdSe_(x)Te_(1-x),wherein x is a number greater than 0 and less than 1. In someembodiments, x is in a range from about 0.01 to about 0.99, and thevalue of “x′ varies across the thickness of the absorber layer 120.

In some embodiments, the absorber layer 120 may further include sulfur.In such instances, at least a portion of the selenium is present in theabsorber layer 120 in the form of a quaternary compound includingcadmium, tellurium, sulfur, and selenium. Further, as noted earlier, insuch instances, the concentration of selenium may vary across athickness of the absorber layer 120.

The absorber layer 120 may be further characterized by the amount ofselenium present. In some embodiments, an average atomic concentrationof selenium in the absorber layer 120 is in a range from about 0.001atomic percent to about 40 atomic percent of the absorber layer 120. Insome embodiments, an average atomic concentration of selenium in theabsorber layer 120 is in a range from about 0.01 atomic percent to about25 atomic percent of the absorber layer 120. In some embodiments, anaverage atomic concentration of selenium in the absorber layer 120 is ina range from about 0.1 atomic percent to about 20 atomic percent of theabsorber layer 120.

As noted, the absorber layer 120 is a component of a photovoltaic device100. In some embodiments, the photovoltaic device 100 includes a“superstrate” configuration of layers. Referring now to FIGS. 3-5 , insuch embodiments, the layer stack 110 further includes a support 111,and a transparent conductive oxide layer 112 (sometimes referred to inthe art as a front contact layer) is disposed on the support 111. Asfurther illustrated in FIGS. 3-5 , in such embodiments, the solarradiation 10 enters from the support 111, and after passing through thetransparent conductive oxide layer 112, the buffer layer 113, andoptional intervening layers (for example, interlayer 114) enters theabsorber layer 120. The conversion of electromagnetic energy of incidentlight (for instance, sunlight) to electron-hole pairs (that is, to freeelectrical charge) occurs primarily in the absorber layer 120.

In some embodiments, the support 111 is transparent over the range ofwavelengths for which transmission through the support 111 is desired.In one embodiment, the support 111 may be transparent to visible lighthaving a wavelength in a range from about 400 nm to about 1000 nm. Insome embodiments, the support 111 includes a material capable ofwithstanding heat treatment temperatures greater than about 600° C.,such as, for example, silica or borosilicate glass. In some otherembodiments, the support 111 includes a material that has a softeningtemperature lower than 600° C., such as, for example, soda-lime glass ora polyimide. In some embodiments certain other layers may be disposedbetween the transparent conductive oxide layer 112 and the support 111,such as, for example, an anti-reflective layer or a bather layer (notshown).

The term “transparent conductive oxide layer” as used herein refers to asubstantially transparent layer capable of functioning as a frontcurrent collector. In some embodiments, the transparent conductive oxidelayer 112 includes a transparent conductive oxide (TCO). Non-limitingexamples of transparent conductive oxides include cadmium tin oxide(Cd₂SnO₄ or CTO); indium tin oxide (ITO); fluorine-doped tin oxide(SnO:F or FTO); indium-doped cadmium-oxide; doped zinc oxide (ZnO), suchas aluminum-doped zinc-oxide (ZnO:Al or AZO), indium-zinc oxide (IZO),and zinc tin oxide (ZnSnO_(x)); or combinations thereof. Depending onthe specific TCO employed and on its sheet resistance, the thickness ofthe transparent conductive oxide layer 112 may be in a range of fromabout 50 nm to about 600 nm, in one embodiment.

The term “buffer layer” as used herein refers to a layer interposedbetween the transparent conductive oxide layer 112 and the absorberlayer 120, wherein the layer 113 has a higher sheet resistance than thesheet resistance of the transparent conductive oxide layer 112. Thebuffer layer 113 is sometimes referred to in the art as a“high-resistivity transparent conductive oxide layer” or “HRT layer”.

Non-limiting examples of suitable materials for the buffer layer 113include tin dioxide (SnO₂), zinc tin oxide (zinc-stannate (ZTO)),zinc-doped tin oxide (SnO₂:Zn), zinc oxide (ZnO), indium oxide (In₂O₃),or combinations thereof. In some embodiments, the thickness of thebuffer layer 113 is in a range from about 50 nm to about 200 nm.

In some embodiments, as indicated in FIGS. 3-5 , the layer stack 110 mayfurther include an interlayer 114 disposed between the buffer layer 113and the absorber layer 120. The interlayer may include a metal species.Non limiting examples of metal species include magnesium, gadolinium,aluminum, beryllium, calcium, barium, strontium, scandium, yttrium,hafnium, cerium, lutetium, lanthanum, or combinations thereof. The term“metal species” as used in this context refers to elemental metal, metalions, or combinations thereof. In some embodiments, the interlayer 114may include a plurality of the metal species. In some embodiments, atleast a portion of the metal species is present in the interlayer 114 inthe form of an elemental metal, a metal alloy, a metal compound, orcombinations thereof. In certain embodiments, the interlayer 114includes magnesium, gadolinium, or combinations thereof.

In some embodiments, the interlayer 114 includes (i) a compoundincluding magnesium and a metal species, wherein the metal speciesincludes tin, indium, titanium, or combinations thereof; or (ii) a metalalloy including magnesium; or (iii) magnesium fluoride; or combinationsthereof. In certain embodiments, the interlayer includes a compoundincluding magnesium, tin, and oxygen. In certain embodiments, theinterlayer includes a compound including magnesium, zinc, tin, andoxygen.

In some embodiments, the absorber layer 120 may function as an absorberlayer in the photovoltaic device 100. The term “absorber layer” as usedherein refers to a semiconducting layer wherein the solar radiation isabsorbed, with a resultant generation of electron-hole pairs. In oneembodiment, the absorber layer 120 includes a p-type semiconductormaterial.

In one embodiment, a photoactive material is used for forming theabsorber layer 120. Suitable photoactive materials include cadmiumtelluride (CdTe), cadmium zinc telluride (CdZnTe), cadmium magnesiumtelluride (CdMgTe), cadmium manganese telluride (CdMnTe), cadmiumtelluride sulfide (CdTeS), zinc telluride (ZnTe), lead telluride (PbTe),Mercury Cadmium Telluride (HgCdTe), lead sulfide (PbS), combinationsthereof. The above-mentioned photo active semiconductor materials may beused alone or in combination. Further, these materials may be present inmore than one layer, each layer having different type of photoactivematerial, or having combinations of the materials in separate layers.

As will be appreciated by one of ordinary skill in the art, the absorberlayer 120 as described herein further includes selenium. Accordingly,the absorber layer 120 may further include a combination of one or moreof the aforementioned photoactive materials and selenium, such as, forexample, cadmium selenide telluride, cadmium zinc selenide telluride,zinc selenide telluride, and the like. In certain embodiments, cadmiumtelluride is used for forming the absorber layer 120. In certainembodiments, the absorber layer 120 includes cadmium, tellurium, andselenium.

In some embodiments, the absorber layer 120 may further include sulfur,oxygen, copper, chlorine, lead, zinc, mercury, or combinations thereof.In certain embodiments, the absorber layer 120 may include one or moreof the aforementioned materials, such that the amount of the materialvaries across a thickness of the absorber layer 120. In someembodiments, one or more of the aforementioned materials may be presentin the absorber layer as a dopant. In certain embodiments, the absorberlayer 120 further includes a copper dopant.

In some embodiments, absorber layer 120 may contain oxygen. In someembodiments, the amount of oxygen is less than about 20 atomic percent.In some instances, the amount of oxygen is between about 1 atomicpercent to about 10 atomic percent. In some instances, for example inthe absorber layer 120, the amount of oxygen is less than about 1 atomicpercent. Moreover, the oxygen concentration within the absorber layer120 may be substantially constant or compositionally graded across thethickness of the respective layer.

In some embodiments, the photovoltaic device 100 may further include ap+-type semiconductor layer 130 disposed on the absorber layer 120, asindicated in FIGS. 3-5 . The term “p+-type semiconductor layer” as usedherein refers to a semiconductor layer having an excess mobile p-typecarrier or hole density compared to the p-type charge carrier or holedensity in the absorber layer 120. In some embodiments, the p+-typesemiconductor layer has a p-type carrier density in a range greater thanabout 1×10¹⁶ per cubic centimeter. The p+-type semiconductor layer 130may be used as an interface between the absorber layer 120 and the backcontact layer 140, in some embodiments.

In one embodiment, the p+-type semiconductor layer 130 includes aheavily doped p-type material including amorphous Si:H, amorphous SiC:H,crystalline Si, microcrystalline Si:H, microcrystalline SiGe:H,amorphous SiGe:H, amorphous Ge, microcrystalline Ge, GaAs, BaCuSF,BaCuSeF, BaCuTeF, LaCuOS, LaCuOSe, LaCuOTe, LaSrCuOS,LaCuOSe_(0.6)Te_(0.4), BiCuOSe, BiCaCuOSe, PrCuOSe, NdCuOS,Sr₂Cu₂ZnO₂S₂, Sr₂CuGaO₃S, (Zn,Co,Ni)O_(x), or combinations thereof. Inanother embodiment, the p+-type semiconductor layer 130 includes ap+-doped material including zinc telluride, magnesium telluride,manganese telluride, beryllium telluride, mercury telluride, arsenictelluride, antimony telluride, copper telluride, elemental tellurium orcombinations thereof. In some embodiments, the p+-doped material furtherincludes a dopant including copper, gold, nitrogen, phosphorus,antimony, arsenic, silver, bismuth, sulfur, sodium, or combinationsthereof.

In some embodiments, the photovoltaic device 100 further includes a backcontact layer 140, as indicated in FIGS. 3-5 . In some embodiments, theback contact layer 140 is disposed directly on the absorber layer 120(embodiment not shown). In some other embodiments, the back contactlayer 140 is disposed on the p+-type semiconductor layer 130 disposed onthe absorber layer 120, as indicated in FIGS. 3-5 .

In some embodiments, the back contact layer 140 includes gold, platinum,molybdenum, tungsten, tantalum, titanium, palladium, aluminum, chromium,nickel, silver, graphite, or combinations thereof. The back contactlayer 140 may include a plurality of layers that function together asthe back contact.

In some embodiments, another metal layer (not shown), for example,aluminum, may be disposed on the back contact layer 140 to providelateral conduction to the outside circuit. In certain embodiments, aplurality of metal layers (not shown), for example, aluminum andchromium, may be disposed on the back contact layer 140 to providelateral conduction to the outside circuit. In certain embodiments, theback contact layer 140 may include a layer of carbon, such as, graphitedeposited on the absorber layer 120, followed by one or more layers ofmetal, such as the metals described above.

As indicated in FIGS. 1-5 , in certain embodiments, the absorber layer120 is disposed directly in contact with the layer stack 110. However,as further noted earlier, in some embodiments, the photovoltaic device100 may include a discontinuous cadmium sulfide layer interposed betweenthe layer stack 110 and the absorber layer 120 (embodiment not shown).In such instances, the coverage of the CdS layer on the underlying layer(for example, interlayer 114 and the buffer layer 113) is less thanabout 20 percent. Further, at least a portion of the absorber layer 120may contact the layer stack 110 through the discontinuous portions ofthe cadmium sulfide layer.

Referring again to FIG. 5 , as indicated, the absorber layer 120 furtherincludes a first region 122 and a second region 124. As furtherillustrated in FIG. 5 , the first region 122 is disposed proximate tothe layer stack 110 relative to the second region 124. In someembodiments, the first region 122 is disposed directly in contact withthe interlayer 114. In some embodiments, the first region 122 isdisposed directly in contact with the buffer layer 113 (embodiment notshown). Further, as discussed earlier, an average atomic concentrationof selenium in the first region 122 is greater than an average atomicconcentration of selenium in the second region 124. In otherembodiments, an average atomic concentration of selenium in the firstregion 122 is lower than an average atomic concentration of selenium inthe second region 124.

In alternative embodiments, as illustrated in FIG. 6 , a photovoltaicdevice 200 including a “substrate” configuration is presented. Thephotovoltaic device 200 includes a layer stack 210 and an absorber layer220 disposed on the layer stack. The layer stack 210 includes atransparent conductive oxide layer 212 disposed on the absorber layer,as indicated in FIG. 6 . The absorber layer 220 is further disposed on aback contact layer 230, which is disposed on a substrate 240. Asillustrated in FIG. 6 , in such embodiments, the solar radiation 10enters from the transparent conductive oxide layer 212 and enters theabsorber layer 220, where the conversion of electromagnetic energy ofincident light (for instance, sunlight) to electron-hole pairs (that is,to free electrical charge) occurs.

In some embodiments, the composition of the layers illustrated in FIG. 6, such as, the substrate 240, the transparent conductive oxide layer212, the absorber layer 220, and the back contact layer 230 may have thesame composition as described above in FIG. 5 for the superstrateconfiguration.

A method of making a photovoltaic device is also presented. In someembodiments, the method generally includes providing an absorber layeron a layer stack, wherein the absorber layer includes selenium, andwherein an atomic concentration of selenium varies across a thickness ofthe absorber layer. With continued reference to FIGS. 1-5 , in someembodiments the method includes providing an absorber layer 120 on alayer stack 110.

In some embodiments, as indicated in FIG. 2 , the step of providing anabsorber layer 120 includes forming a first region 122 and a secondregion 124 in the absorber layer 120, the first region 122 disposedproximate to the layer stack 110 relative to the second region 124. Asnoted earlier, in some embodiments, an average atomic concentration ofselenium in the first region 122 is greater than an average atomicconcentration of selenium in the second region 124.

The absorber layer 120 may be provided on the layer stack 110 using anysuitable technique. In some embodiments, the step of providing anabsorber layer 120 includes contacting a semiconductor material with aselenium source. The terms “contacting” or “contacted” as used hereinmeans that at least a portion of the semiconductor material is exposedto, such as, in direct physical contact with a suitable selenium sourcein a gas, liquid, or solid phase. In some embodiments, a surface of theabsorber layer may be contacted with the suitable selenium source, forexample using a surface treatment technique. In some other embodiments,the semiconductor material may be contacting with a suitable seleniumsource, for example, using an immersion treatment.

In some embodiments, the semiconductor material includes cadmium.Non-limiting examples of a suitable semiconductor material includecadmium telluride (CdTe), cadmium zinc telluride (CdZnTe), cadmiummagnesium telluride (CdMgTe), cadmium manganese telluride (CdMnTe),cadmium sulfur telluride (CdSTe), zinc telluride (ZnTe), lead telluride(PbTe), lead sulfide (PbS), mercury cadmium telluride (HgCdTe), orcombinations thereof. In certain embodiments, the semiconductor materialincludes cadmium and tellurium.

The term “selenium source” as used herein refers to any materialincluding selenium. Non-limiting examples of a suitable selenium sourceinclude elemental selenium, cadmium selenide, oxides of cadmiumselenide, such as, for example, cadmium selenite (CdSeO₃), hydrogenselenide, organo-metallic selenium, or combinations thereof.

The portion of the semiconductor material contacted with the seleniumsource may depend, in part, on the physical form of the selenium sourceduring the contacting step. In some embodiments, the selenium source isin the form of a solid (for example, a layer), a solution, a suspension,a paste, vapor, or combinations thereof. Thus, by way of example, insome embodiments, for example, the selenium source may be in the form ofa solution, and the method may include soaking at least a portion of thesemiconductor material in the solution.

In some embodiments, the selenium source may be in the form a vapor, andthe method may include depositing the selenium source using a suitablevapor deposition technique. In some embodiments, for example, theabsorber layer 120 may be heat treated in the presence of a seleniumsource (for example, selenium vapor) to introduce selenium into at leasta portion of the absorber layer 120.

In some embodiments, for example, the selenium source may be in the formof a layer, and the method may include depositing a selenium sourcelayer on the semiconductor material, or, alternatively, depositing thesemiconductor material on a layer of the selenium source. In some suchembodiments, the method may further include subjecting the semiconductormaterial to one or more post-processing steps to introduce the seleniuminto the semiconductor material.

Referring now to FIG. 7 , in some embodiments, the step of providing anabsorber layer includes (a) disposing a selenium source layer 125 on thelayer stack 110; (b) disposing an absorber layer 120 on the seleniumsource layer 125; and (c) introducing selenium into at least a portionof the absorber layer 120. It should be noted, that the steps (b) and(c) may be performed sequentially or simultaneously.

In some embodiments, the selenium source layer 125 may be disposed onthe layer stack 110 using any suitable deposition technique, such as,for example, sputtering, sublimation, evaporation, or combinationsthereof. The deposition technique may depend, in part, on one or more ofthe selenium source material, the selenium source layer 125 thickness,and the layer stack 110 composition. In certain embodiments, theselenium source layer 125 may include elemental selenium and theselenium source layer 125 may be formed by evaporation. In certainembodiments, the selenium source layer 125 may include cadmium selenide,and the selenium source layer 125 may be formed by sputtering,evaporation, or sublimation.

The selenium source layer may include a single selenium source layer ora plurality of selenium source layers. The selenium source may be thesame or different in the plurality of source layers. In someembodiments, the selenium source layer includes a plurality of seleniumsource layers, such as, for example, a stack of elemental selenium layerand a cadmium selenide layer, or vice versa.

The selenium source layer 125 may have a thickness in a range from about1 nanometer to about 1000 nanometers. In some embodiments, the seleniumsource layer 125 has a thickness in a range from about 10 nanometers toabout 500 nanometers. In some embodiments, the selenium source layer 125has a thickness in a range from about 15 nanometers to about 250nanometers.

As noted, the method further includes disposing an absorber layer 120 onthe selenium source layer 125. In some embodiments, the absorber layer120 may be deposited using a suitable method, such as, close-spacesublimation (CSS), vapor transport deposition (VTD), ion-assistedphysical vapor deposition (IAPVD), radio frequency or pulsed magnetronsputtering (RFS or PMS), chemical vapor deposition (CVD), plasmaenhanced chemical vapor deposition (PECVD), or electrochemicaldeposition (ECD).

The method further includes introducing selenium into at least a portionof the absorber layer 120. In some embodiments, the method includesintroducing selenium into at least a portion of the absorber layer 120such that a concentration of selenium varies across the thickness of theabsorber layer 120.

In some embodiments, at least a portion of selenium is introduced in theabsorber layer 120 simultaneously with the step of disposing theabsorber layer 120. In some embodiments, at least a portion of seleniummay be introduced after the step of disposing the absorber layer 120,for example, during the cadmium chloride treatment step, during thep+-type layer formation step, during the back contact formation step, orcombinations thereof.

In some embodiments, the step of providing an absorber layer 120includes co-depositing a selenium source material and a semiconductormaterial. Suitable non-limiting examples of co-deposition includeco-sputtering, co-sublimation, or combinations thereof. Non-limitingexamples of a suitable selenium source material in such instanceincludes elemental selenium, cadmium selenide, hydrogen selenide,cadmium telluride selenide, or combinations thereof. Thus, by way ofexample, in some embodiments, an absorber layer 120 may be provided bydepositing the semiconductor material in the presence of selenium source(for example, selenium containing vapor or hydrogen selenide vapor).

In some embodiments, the absorber layer 120 may be provided bysputtering from a single target (for example, cadmium selenide telluridetarget) or a plurality of targets (for example, cadmium telluride andcadmium selenide targets). As will be appreciated by one of ordinaryskill in the art, the concentration of selenium in the absorber layer120 may be varied by controlling one or both of target(s) compositionand sputtering conditions.

As noted earlier, the photovoltaic device 100 and the layer stack 110may further include one or more additional layers, for example, asupport 111, a transparent conductive oxide layer 112, a buffer layer113, an interlayer 114, a p+-type semiconductor layer 130, and a backcontact layer 140, as depicted in FIGS. 3-5 .

As understood by a person skilled in the art, the sequence of disposingthe three layers or the whole device may depend on a desirableconfiguration, for example, “substrate” or “superstrate” configurationof the device.

In certain embodiments, a method for making a photovoltaic 100 insuperstrate configuration is described. Referring now to FIGS. 3-5 , insome embodiments, the method further includes disposing the transparentconductive oxide layer 112 on a support 111. The transparent conductiveoxide layer 112 is disposed on the support 111 by any suitabletechnique, such as sputtering, chemical vapor deposition, spin coating,spray coating, or dip coating. Referring again to FIGS. 3-5 , in someembodiments, a buffer layer 113 may be deposited on the transparentconductive oxide layer 112 using sputtering. The method may furtherincluding disposing an interlayer 114 on the buffer layer 113 to form alayer stack 110, as indicated in FIG. 4 .

The method further includes providing an absorber layer 120 on the layerstack 110, as described in detail earlier. In some embodiments, a seriesof post-forming treatments may be further applied to the exposed surfaceof the absorber layer 120. These treatments may tailor the functionalityof the absorber layer 120 and prepare its surface for subsequentadhesion to the back contact layer(s) 140. For example, the absorberlayer 120 may be annealed at elevated temperatures for a sufficient timeto create a quality p-type layer. Further, the absorber layer 120 may betreated with a passivating agent (e.g., cadmium chloride) and atellurium-enriching agent (for example, iodine or an iodide) to form atellurium-rich region in the absorber layer 120. Additionally, coppermay be added to absorber layer 120 in order to obtain a low-resistanceelectrical contact between the absorber layer 120 and a back contactlayer(s) 140.

Referring again to FIGS. 3-5 , a p+-type semiconducting layer 130 may befurther disposed on the absorber layer 120 by depositing a p+-typematerial using any suitable technique, for example PECVD or sputtering.In an alternate embodiment, as mentioned earlier, a p+-typesemiconductor region may be formed in the absorber layer 120 bychemically treating the absorber layer 120 to increase the carrierdensity on the back-side (side in contact with the metal layer andopposite to the window layer) of the absorber layer 120 (for example,using iodine and copper). In some embodiments, a back contact layer 140,for example, a graphite layer may be deposited on the p+-typesemiconductor layer 130, or directly on the absorber layer 120(embodiment not shown). A plurality of metal layers may be furtherdeposited on the back contact layer 140.

One or more of the absorber layer 120, the back contact layer 140, orthe p+-type layer 130 (optional) may be further heated or subsequentlytreated (for example, annealed) after deposition to manufacture thephotovoltaic device 100.

In some embodiments, other components (not shown) may be included in theexemplary photovoltaic device 100, such as, buss bars, external wiring,laser etches, etc. For example, when the device 100 forms a photovoltaiccell of a photovoltaic module, a plurality of photovoltaic cells may beconnected in series in order to achieve a desired voltage, such asthrough an electrical wiring connection. Each end of the seriesconnected cells may be attached to a suitable conductor such as a wireor bus bar, to direct the generated current to convenient locations forconnection to a device or other system using the generated current. Insome embodiments, a laser may be used to scribe the deposited layers ofthe photovoltaic device 100 to divide the device into a plurality ofseries connected cells.

EXAMPLES Comparative Example 1 Method of Manufacturing a CadmiumTelluride Photovoltaic Device Including a CdS/CdTe Layer Stack

A cadmium telluride photovoltaic device was made by depositing severallayers on a cadmium tin oxide (CTO) transparent conductive oxide(TCO)-coated substrate. The substrate was a 1.4 millimeters thick PVN++glass, which was coated with a CTO transparent conductive oxide layerand a thin high resistance transparent zinc tin oxide (ZTO) bufferlayer. A magnesium-containing capping layer was then deposited on theZTO buffer layer to form an interlayer. The window layer (30 nanometersthick) containing cadmium sulfide (CdS:O, 5 molar % oxygen in the CdSlayer) was then deposited on the interlayer by DC sputtering followed bydeposition of cadmium telluride (CdTe) layer at 550° C., and backcontact formation.

Comparative Example 2 Method of Manufacturing a Cadmium TelluridePhotovoltaic Device Including a CdTe Layer and No CdS Layer

A cadmium telluride photovoltaic device was made by depositing severallayers on a cadmium tin oxide (CTO) transparent conductive oxide(TCO)-coated substrate. The substrate was a 1.4 millimeters thick PVN++glass, which was coated with a CTO transparent conductive oxide layerand a thin high resistance transparent zinc tin oxide (ZTO) bufferlayer. A magnesium-containing capping layer was then deposited on theZTO buffer layer to form an interlayer. This was followed by depositionof cadmium telluride (CdTe) layer at 550° C., and back contactformation.

Example 1 Method of Manufacturing a Cadmium Telluride PhotovoltaicDevice Including a Graded CdTeSe Layer and No CdS Layer

The method of making the photovoltaic device was similar to theComparative Example 2, except after the step of interlayer formation, a120 to 140 nanometers thick CdSe layer was sputtered on the interlayer,followed by deposition of CdTe layer (band gap=1.5 eV) on the CdSe layer(band gap=1.74 eV), and back contact formation.

During device processing, the absorption edge of the composite CdSe/CdTeabsorber appears to shift to energies that are lower than either pureCdTe or pure CdSe, as evidenced by a red shifted absorption edge asmeasured using wavelength dependence of the quantum efficiency (QE) ofthe solar cells (FIG. 12 ). This is consistent with extensiveintermixing of the Te and Se within the absorber layer 120 therebycreating a lower band gap alloy. Further evidence of cadmium selenidetelluride alloy formation is seen in photo-luminescence (PL) emissionspectroscopy (FIG. 13 ), showing a band gap reduced substantially belowComparative Examples 1 and 2. The observed QE and PL shifts to lowerenergy are inconsistent with the CdSe remaining entirely as a separatephase.

The SIMS profile (FIGS. 10 and 11 ) indicates that the concentration ofSe is not homogenous throughout the absorbing layer 120, indicating thatthe alloy detected by the QE and PL spectroscopy methods is graded, withhigher concentrations of Se alloy located near the front side of theabsorber layer 120. As illustrated in FIG. 10 , a substantial portion ofselenium is incorporated in the CdTe layer after device formation.Further, the concentration of selenium varies across the thickness ofCdTe layer, with a higher amount of selenium present near the frontinterface.

FIG. 8 illustrates the device performance parameters (normalized withrespect to Comparative Example 1) for devices prepared in ComparativeExample 2 and Example 1. As illustrated in FIG. 8 , the deviceperformance parameters showed comparable or better performance for thedevices with a graded CdTeSe layer (Example 1) when compared to thedevice without a CdTeSe layer (Comparative Example 1). Thus, devicesthat have a graded CdTeSe layer and are substantially free of a CdSlayer (Example 1) showed comparable performance to devices with theCdS/CdTe layer stack (Comparative Example 1), and significantly improvedperformed when compared to the devices including just the CdTe layer(Comparative Example 2).

FIG. 9 shows the time-resolved photoluminescence (TRPL) lifetime curvesfor Example 1 amd Comparative Examples 1 and 2. Time resolvedphotoluminescence (TRPL) spectroscopy was performed using a Picoquantsub-nanosecond pulsed laser operating at near 640 nm. The pulserepetition rate of the laser was set to 2.5 MHz, and the typical laserpower density was estimated to be about 10¹³ photons/cm², as estimatedby a measurement of the typical laser output power and focusingcharacteristics. The actual photon flux incident on the sample may varyfrom this value. The luminescence light was filtered by a long-passglass filter with a cut off wavelength of about 700 nm to eliminatestray laser excitation light and then coupled into a monochrometerequipped with a grating and a slit set so that light of 840 nm with awindow of about +/−1 nm was detected. The filtered light from themonochrometer was then coupled to a cooled multi-channel photomultiplier tube (Hamamatsu R3809U series). Individual photon events werecounted and timed using the time-correlated photon counting electronicssupplied by Edinburgh instruments.

The appended claims are intended to claim the invention as broadly as ithas been conceived and the examples herein presented are illustrative ofselected embodiments from a manifold of all possible embodiments.Accordingly, it is the Applicants' intention that the appended claimsare not to be limited by the choice of examples utilized to illustratefeatures of the present invention. As used in the claims, the word“comprises” and its grammatical variants logically also subtend andinclude phrases of varying and differing extent such as for example, butnot limited thereto, “consisting essentially of” and “consisting of.”Where necessary, ranges have been supplied; those ranges are inclusiveof all sub-ranges there between. It is to be expected that variations inthese ranges will suggest themselves to a practitioner having ordinaryskill in the art and where not already dedicated to the public, thosevariations should where possible be construed to be covered by theappended claims. It is also anticipated that advances in science andtechnology will make equivalents and substitutions possible that are notnow contemplated by reason of the imprecision of language and thesevariations should also be construed where possible to be covered by theappended claims.

The invention claimed is:
 1. A photovoltaic device, comprising: atransparent layer stack; an absorber layer disposed directly in contactwith the layer stack at a front interface of the absorber layer, theabsorber layer consisting of a thickness between the front interface ofthe absorber layer and a back interface of the absorber layer, wherein:the front interface of the absorber layer is closer to the transparentlayer stack than the back interface of the absorber layer, the absorberlayer comprises a compound of cadmium, selenium, and tellurium, anatomic concentration of selenium varies across the thickness of theabsorber layer, the atomic concentration of selenium is greater at thefront interface of the absorber layer relative to the back interface ofthe absorber layer, a band gap of the absorber layer at the frontinterface of the absorber layer is lower than the band gap of theabsorber layer at the back interface of the absorber layer, the absorberlayer consisting of: a first region and a second region, wherein: thefirst region extends from the front interface of the absorber layer tothe second region, the second region extends from the first region tothe back interface of the absorber layer, the first region has acontinuous uniform thickness between 100 nanometers to 3000 nanometersthick, the second region has a continuous uniform thickness between 100nanometers to 3000 nanometers thick, and a ratio of an average atomicconcentration of selenium in the first region to an average atomicconcentration of selenium in the second region is greater than 10; and ap+ type semiconducting layer, wherein: the absorber layer is between thetransparent layer stack and the p+ type semiconducting layer, and the p+type semiconducting layer comprises at least one of: zinc telluride,magnesium telluride, manganese telluride, beryllium telluride, mercurytelluride, arsenic telluride, antimony telluride, or copper telluride.2. The photovoltaic device of claim 1, wherein the absorber layerfurther comprises sulfur, oxygen, copper, chlorine, or combinationsthereof.
 3. The photovoltaic device of claim 1, wherein at least aportion of selenium is present in the absorber layer in the form of aternary compound, a quaternary compound, or combinations thereof.
 4. Thephotovoltaic device of claim 1, wherein an average atomic concentrationof selenium in the absorber layer is in a range from about 0.001 atomicpercent to about 40 atomic percent of the absorber layer.
 5. Thephotovoltaic device of claim 4, wherein the average atomic concentrationof selenium in the absorber layer is in a range from 0.001 atomicpercent to 20 atomic percent of the absorber layer.
 6. The photovoltaicdevice of claim 1, wherein the absorber layer comprises a plurality ofgrains separated by grain boundaries, and wherein an average atomicconcentration of selenium in the grain boundaries is higher than anaverage atomic concentration of selenium in the grains.
 7. Thephotovoltaic device of claim 1, wherein the layer stack comprises: atransparent conductive layer disposed on a support; and a buffer layerdisposed between the transparent conductive layer and the absorberlayer.
 8. The photovoltaic device of claim 7, wherein the layer stackfurther comprises an interlayer disposed between the buffer layer andthe absorber layer.
 9. The photovoltaic device of claim 8, wherein theabsorber layer is in direct contact with the interlayer.
 10. Thephotovoltaic device of claim 1, wherein the photovoltaic device has nowindow layer and is completely free of a cadmium sulfide layer.
 11. Thephotovoltaic device of claim 1, wherein: the first region is between 200nanometers to 1500 nanometers thick, the second region is between 200nanometers to 1500 nanometers thick.
 12. A photovoltaic device,comprising: a layer stack; and an absorber layer disposed directly incontact with the layer stack at a front interface of the absorber layer,the absorber layer consisting of a thickness between the front interfaceand a back interface of the absorber layer, wherein: the absorber layercomprises a compound of cadmium, selenium, and tellurium, an atomicconcentration of selenium varies continuously across the thickness ofthe absorber layer, the atomic concentration of selenium is greater atthe front interface of the absorber layer relative to the back interfaceof the absorber layer, the photovoltaic device has no window layer andis substantially free of a cadmium sulfide layer, and the photovoltaicdevice includes a layer comprising: cadmium tin oxide, indium tin oxide;fluorine-doped tin oxide; indium-doped cadmium-oxide; doped zinc oxide,indium-zinc oxide, or zinc tin oxide.
 13. The photovoltaic device ofclaim 12, wherein the absorber layer further comprises oxygen, andwherein the amount of oxygen in the absorber layer is less than 1 atomicpercent.
 14. The photovoltaic device of claim 12, wherein an averageatomic concentration of selenium in the absorber layer is in a rangefrom 0.001 atomic percent to 20 atomic percent of the absorber layer.15. The photovoltaic device of claim 12, wherein the photovoltaic devicefurther comprises a p+ type semiconducting layer, wherein: the absorberlayer is between the transparent layer stack and the p+ typesemiconducting layer, and the p+ type semiconducting layer comprises atleast one of: zinc telluride, magnesium telluride, manganese telluride,beryllium telluride, mercury telluride, arsenic telluride, antimonytelluride, or copper telluride.
 16. A photovoltaic device, comprising: atransparent layer stack; and an absorber layer in direct contact withthe transparent layer stack at a front interface of the absorber layer,wherein: the absorber layer consists of a thickness of the absorberlayer between the front interface of the absorber layer and a backinterface of the absorber layer, the front interface of the absorberlayer is closer to the transparent layer stack than the back interfaceof the absorber layer, the absorber layer comprises cadmium, selenium,and tellurium, an atomic concentration of selenium varies across thethickness of the absorber layer, the atomic concentration of selenium isgreater at the front interface of the absorber layer relative to theback interface of the absorber layer, the absorber layer is a p-typelayer, the absorber layer consists of a first region and a secondregion, the first region extends from the front interface of theabsorber layer to the second region, the second region extends from thefirst region to the back interface of the absorber layer, the firstregion has a continuous uniform thickness between 100 nanometers to 3000nanometers thick, the second region has a continuous uniform thicknessbetween 100 nanometers to 3000 nanometers thick, a ratio of an averageatomic concentration of selenium in the first region to an averageatomic concentration of selenium in the second region is greater than10, and the photovoltaic device has no window layer and is completelyfree of a cadmium sulfide layer.
 17. The photovoltaic device of claim16, wherein an average atomic concentration of selenium in the absorberlayer is in a range from 0.001 atomic percent to 20 atomic percent ofthe absorber layer.